1. Field of the Invention
The present invention relates to a process for preparing acrylic acid by catalytic gas phase oxidation, in particular to recovery of acrylic acid from the acid water produced.
2. Description of the Background
Acrylic acid can be prepared from propene by catalytic gas phase oxidation. The propene to be oxidized can be mixed with a diluent gas or with a circulated gas containing unreacted starting materials. The gas-phase oxidation reaction mixture is passed to an absorption column to separate off the acrylic acid. There, the acrylic acid is substantially separated off from the mixture of reaction products using solvents such as lactams, for example methylpyrrolidone, or other organic acids, for example ethylhexanoic acid. The non-absorbed components are taken off from the absorption column in the gaseous state and fed to a condensation stage. The condensible and condensed part of this gas mixture is separated off and led off as what is termed acid water. The uncondensible gaseous part, in contrast, is in part recycled to the gas phase oxidation and forms the circulated gas. The acid water is generally incinerated. Thus, although many low- and medium-boilers are removed from the production cycle, acrylic acid present in the acid water is also destroyed.
This loss of acrylic acid can be avoided by distilling the acid water or extracting it with suitable compounds. Suitable compounds for extracting acrylic acid from aqueous solutions are specified by I. M. Korenman et al. in "Distribution of acrylic acid between organic solvents and water" (translation by Consultants Bureau, New York, from Zhurnal Prikladnoi Khimii, Vol. 45, No. 5, May 1972, pp. 1078-1082), including dimethyl phthalate. Distillation has the disadvantage of a high energy consumption, and extraction leads to a not inconsiderable additional use of equipment, since, in addition to extracting the acid water, it is necessary to distil or strip the solvent from the extract phase. This cycle for recovering acrylic acid from the acid water is complicated, especially, by the need to avoid as far as possible carrying over the solvent used there into the preparation cycle, since the presence of further organic solvents in the preparation cycle can adversely effect the quantitative yield and the quality of the acrylic acid present. It is highly complicated to exclude adverse effects of this type reliably.